Electrophotographic marking is a well known method of copying or printing documents or other substrates. Electrophotographic marking is typically performed by exposing a light image of an original document onto a substantially uniformly charged photoreceptor. That light image discharges the photoreceptor so as to create an electrostatic latent image of the original on the photoreceptor's surface. Toner particles are then deposited onto the latent image so as to form a toner image. That toner image is then transferred from the photoreceptor, either directly or after an intermediate transfer step, onto a marking substrate such as a sheet of paper. The transferred toner powder image is then fused to the marking substrate using heat and/or pressure. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the creation of another image.
While many types of light exposure systems have been developed, a commonly used system is the raster output scanner (ROS). A raster output scanner is comprised of a laser beam source, a modulator for modulating the laser beam (which, as in the case of a laser diode, may be the source itself) such that the laser beam contains image information, a rotating polygon having at least one reflective surface, input optics that collimate the laser beam, and output optics which focus the laser beam into a spot on the photoreceptor and which correct for various optical problems such as wobble. The laser source, modulator, and input optics produce a collimated laser beam which is directed toward the polygon. As the polygon rotates the reflective surface(s) causes the laser beam to be swept along a scan plane. The swept laser beam passes through the output optics and is reflected by the mirror(s) so as to produce a sweeping spot on a charged photoreceptor. The sweeping spot traces a scan line across the photoreceptor. Since the charged photoreceptor moves in a direction which is substantially perpendicular to the scan line, the sweeping spot raster scans the photoreceptor. By suitably modulating the laser beam a desired latent image can be produced on the photoreceptor.
To assist the understanding of the present invention several things should be further described and highlighted. First, most prior art electrophotographic printing machines are single resolution devices; that is, they produce an image at N number of spots per inch in the cross-scan direction (the direction which the photoreceptor moves), where N is typically 300, 600 or 800. But whatever N is, it is fixed. While single resolution printing machines are relatively straightforward, they may not be optimal. For example, when printing a bit-map which represents an image of M spots per inch on a prior art machine which has a resolution of N spots per inch, where M is not equal to N, software resolution conversion is required before printing. Not only does such software conversion require significant time and computer resources, but bit round-off errors frequently occur. Therefore, an electrophotographic printing machine having variable resolution would be advantageous.
When attempting to implement a variable resolution electrophotographic printing machine it quickly becomes obvious that one approach to achieving variable resolution is to change the size of the spot produced on the photoreceptor by the laser beam. Changing the dimension of the spot in the fast scan direction is relatively easy. In a machine with a fixed scan rate the laser spot images an area having length which is predominately controlled by the time duration that the laser is turned on. To write at a lower resolution the laser can be turned on for longer periods of time. However, since the cross-scan dimension of the image area illuminated by the spot is controlled by the cross-scan dimension of the spot, controlling the resolution in the cross-scan dimension is much more difficult. Even in the fast scan direction is may be beneficial to be able to electronically control the length of the spot without changing the laser on time. Therefore, a technique of controlling the dimensions of the spot on the photoreceptor would be advantageous.